The present invention relates to a process wherein water is separated from pyrolysis gasoline obtained from a steam cracking step using a coalescer for the water separation. The present invention further relates to a device comprising a coalescer for the water separation from pyrolysis gasoline.
A coalescer is a technological device performing coalescence. Coalescers are primarily used to separate two liquid phase systems like dispersions or preferably emulsions into their components via various processes. Coalescers are operated reverse to an emulsifier. Besides commonly used electronic coalescers, one main group of coalescers are mechanical coalescers. Mechanical coalescers use surfaces on which droplets coalesce. Apparatuses typically used are filter cartridges, mesh materials or vessels with different fillings. The use of mechanical coalescers is e.g. described in “Siemens Miltronics Process Instruments Inc., Case Study, 2006” and “Chem.-Ing.-Tech., 1976, No 3, pages 177 to 189”.
Steam cracking is a petrochemical process in which saturated hydrocarbons are broken down into smaller, mostly unsaturated hydrocarbons. Steam cracking is the principal industrial method for producing lighter olefins like ethylene and propylene as disclosed for example in EP-A 1 302 525. Pyrolysis gasoline is a typical product which can be obtained from a steam cracking process. However, pyrolysis gasoline may also be considered as an intermediate, since it is usually subjected to further processing steps such as hydrogenation and/or separation into its components.
The article of H.-M. Allmann et al. provided at the DGMK-conference “Selective hydrogenation and dehydrogenation” on Nov. 11/12, 1993, Kassel/Germany (total of 29 pages) provides an overview on selective hydrogenations and purifications in the steam cracker downstream treatment. However, said article is silent on the presence of water during the steam cracking process or the following purification and hydrogenation steps.
In case of (free) water is present in the organic pyrolysis gasoline stream the hydrogenation catalyst may be partially or completely deactivated. Generally, the activity and life time of a fixed bed catalyst for pyrolysis gasoline hydrogenation depends on the amount of water contacting the catalyst surface. Thus, the presence of water in the pyrolysis gasoline stream has the consequence that the turnover number connected with the activity of the hydrogenation catalyst drops down and the hydrogenation catalyst needs to be regenerated more often. This means an economical and environmental disadvantage. Besides, this small water amount cannot be separated by usual methods—like phase separation by gravity—since this water amount is dispensed into small water drops in the pyrolysis gasoline stream. Also methods like e.g. distillations are ineffective for reducing the amount of water in pyrolysis gasoline obtained from a steam cracking step.
The article of Lee Siang Hua (“Titan's Experience in Physical Separation Devices”; prepared for the 15th Symposium of Chemical Engineers, Hyatt Hotel, Johor Bahru, Malaysia, Sep. 12, 2001; total of 7 pages) provides an overview on separation devices to be employed for removing, for example, water from organic compound streams obtained from a steam cracking process. Four different types of physical separation devices are discussed. In order to perform liquid-liquid separations, the operation may either be based on gravity separation or on coalescing separation in the oil-water coalescer to separate dissolved hydrocarbon in quench water or to remove water from raw pyrolysis gasoline. The coalescing separation may be employed, where small particles of one liquid phase must be separated or removed from a large quantity of another liquid phase. The disclosed coalescers are wire mesh coalescers contained in a horizontal vessel.
The article of Pall Corporation (“Liquid/Liquid coalescer applications: Ethylene processing-dilution steam system”; edited in 1997 by Pall Corporation; total of 2 pages) discloses a process for obtaining ethylene from pyrolysis gasoline. A total of four coalescers are employed at different positions and for different purposes within said ethylene process. One coalescer is, for example, employed to remove gasoline from water out of a first stream obtained from an oil/water separator. Another coalescer is employed to remove water from gasoline out of a second stream obtained from said oil/water separator. However, said article of Pall Corporation is silent on the material of said coalescers.
It is an object of the present invention to provide a method for the efficient separation of water from pyrolysis gasoline obtained from a steam cracking step, especially for removing the remaining water left over after a conventional phase separation step.
The problem of the present invention is solved by providing a process for reducing the water content in pyrolysis gasoline obtained from a steam cracking step, characterized in that water is separated from pyrolysis gasoline via at least one coalescer, which is made of metal, fibre glass or as a combination of metal and fibre glass.
The advantage in using a coalescer made of metal and/or fibre glass for the water separation from pyrolysis gasoline obtained from a steam cracking step is that water can be removed very efficiently. The total water content within a pyrolysis gasoline stream is the sum of the dissolved water and the entrained water. Only the entrained water can be removed from the pyrolysis gasoline by coalescence. This advantage is more significant when employing the coalescer after a conventional phase separation step for removing the main amount of water.
Furthermore, when the coalescer is used before the hydrogenation step, this hydrogenation step has a higher conversion. Therefore, the hydrogenation catalyst has a longer lifetime, since it needs to be regenerated less often. In addition, the chemical structure of the catalyst is changed by entrained water, which is avoided by water separation before hydrogenation.
It is often the case that additional streams like purchased partially hydrogenated pyrolysis gasoline or streams from reformers are mixed into the pyrolysis gasoline before the second hydrogenation step. In these cases, water separation before the hydrogenation step provides advantages to protect the catalysts similar to a separation before first hydrogenation step.